In urban centers, electrical power is delivered via insulated power cables that are installed in underground pipes, conduits, and tunnels to traverse legs between stations that vary between a few hundred meters and about 20 kilometers in length. In some instances, the insulated power cables are installed in “station exits” between a station and riser pole or other situations where it is necessary to connect an underground cable to an overhead line, in which case the length can be as short as 100 ft. These power cables can also be laid in trenches excavated and backfilled with native soils or select materials or laid directly upon or jetted below the seabed. Each power cable carries a single phase, and three power cables are often grouped together forming a circuit to transmit alternating currents. The power cables installed on electric circuits operating at voltages equal to or higher than 5 kV include a metallic shield or sheath that is applied over the insulated core in order the confine the electric field to the cable and to provide a conductive path for system fault currents to return to their source. During operation, the current flowing in the cable conductor causes standing voltages to form and currents to flow along the sheaths of the power cables. These sheath currents raise the operating temperature of the cable and thus limit the current-carrying capacity of the cable and reduce the efficiency of power delivery.
In an effort to reduce these standing voltages and currents, the sheaths are divided into sheath sections that are then coupled via bonding leads in enclosures called link boxes that are configured to eliminate or reduce the magnitude of these standing voltages and currents. The bonding leads are brought into the link boxes via apertures in their housings and then the conductors thereof are exposed and coupled to internal connectors within the link boxes. These link boxes also serve to balance the voltage and currents across the sheaths to maximize the current carrying capacity of the power transmission system. Additionally, in certain applications, the link boxes may also contain voltage limiting devices known as surge voltage limiters (“SVLs”) to limit voltages developed between the sheath and ground to protect the equipment and connected cable system from failure due to lightning strikes and other electrical system disturbances.
As these link boxes are often subjected to damp conditions and, in some cases, may be exposed to intermittent or prolonged submersion in their underground installation locations, such as a manhole, vault or hand hole, the housings and related penetrations must be sealed to inhibit liquid or moisture ingress. Such ingress would at least partially negate the effectiveness of the bonding arrangement afforded by the link boxes, enabling electric current to flow in the cable sheaths thus increasing the operating temperature and reducing the reliability of the power cables.
The link boxes are designed to be opened and closed as required during installation and maintenance activities to periodically inspect the housing, links, and voltage limiters, and intermittently test the insulating properties of the electrical transmission cable system and/or to facilitate fault locating of the electrical transmission cable system. In order to do so, the link boxes are opened and linkages between the bonding cables are reconfigured to enable such testing. Upon completion of the testing, the linkages are typically restored to their normal configuration prior to opening of the link boxes and the link boxes are resealed to protect the exposed conductors of the bonding leads and internal components.